debugcommands: introduce debugrevlogindex (BC) `hg debugindex` was originally invented for displaying revlog index data and is highly tailored towards that. e.g. it accepts a --format option to display index data for a particular revlog version and displays things like offset and length. As we support non-revlog storage, it makes sense for `hg debugindex` to display generic index data and for there to exist storage-specific or storage-aware debug* commands for dumping non-generic index data. This commit effectively renames `hg debugindex` to `hg debugrevlogindex` and replaces `hg debugindex` with a version that is storage agnostic. Tests using --format have been updated to use `hg debugrevlogindex`. Output is backwards compatible. The replacement command uses the formatter, which means output can be templatized. At some point, we may want to tweak output (e.g. to add the revision size). But I don't feel like taking a bigger BC break at the moment. The renamed command/function had to be moved because check-code enforces alphabetical ordering of commands in this file. Differential Revision: https://phab.mercurial-scm.org/D4358

--- a/tests/test-completion.t	Mon Aug 20 23:08:57 2018 +0000
+++ b/tests/test-completion.t	Tue Aug 21 00:01:54 2018 +0000
@@ -111,6 +111,7 @@
   debugrebuildfncache
   debugrename
   debugrevlog
+  debugrevlogindex
   debugrevspec
   debugserve
   debugsetparents
@@ -279,7 +280,7 @@
   debugfsinfo: 
   debuggetbundle: head, common, type
   debugignore: 
-  debugindex: changelog, manifest, dir, format
+  debugindex: changelog, manifest, dir, template
   debugindexdot: changelog, manifest, dir
   debuginstall: template
   debugknown: 
@@ -298,6 +299,7 @@
   debugrebuildfncache: 
   debugrename: rev
   debugrevlog: changelog, manifest, dir, dump
+  debugrevlogindex: changelog, manifest, dir, format
   debugrevspec: optimize, show-revs, show-set, show-stage, no-optimized, verify-optimized
   debugserve: sshstdio, logiofd, logiofile
   debugsetparents: